1. Field of the Invention
The present invention relates to the technology of driving a rotating electrical machine by supplying a current for each phase.
2. Description of the Related Art
Normally, an inverter device is used in driving a rotating electrical machine such as a 3-phase motor. The inverter device is normally provided with two switching elements for each phase (a total of 6 elements). Thus, the inverter device controls the driving voltages of three phases by driving on and off the six switching elements, thereby generating a rotating magnetic field for the rotating electrical machine. The inverter device of the rotating electrical machine is to drive the rotating electrical machine using the switching elements.
As a method for controlling the level of the voltage to be applied to a rotating electrical machine and the frequency, PWM control drives on and off the switching element for each phase, and applies a voltage of any pulse width. The PWM control normally compares a carrier signal with a specified command voltage value, and sets the level of the driving voltage at an H (high) or an L (low) level (carrier synchronization method) depending on the comparison result.
In the PWM control, the switching frequency at which the switching elements are driven on and off depends on the carrier frequency. When a switching loss is considered, it is preferable that the switching frequency is lower. The switching frequency can be lower by setting a lower carrier frequency. Therefore, some conventional inverter devices of rotating electrical machine change the carrier frequency depending on the inclination of the reference sine wave signal used in providing a command voltage value (Japanese Laid-open Patent Publication No. 2010-35260, hereafter referred to as the “patent document 1”).
In the conventional inverter device described in the patent document 1, the carrier frequency is changed depending on the inclination of the reference sine wave signal. Therefore, complicated control is performed. In addition, since the frequency of an output waveform is to be fixed, the synchronous PWM control cannot be executed when the frequency is variable. Therefore, it is also important to realize the switching loss without narrowing the width of options in the control method while suppressing the complexity of the control.
Other reference documents can be Japanese Laid-open Patent Publication No. 2007-228745 and Japanese Laid-open Patent Publication No. 9-47026.